Stepped-impedance directional coupler

ABSTRACT

One or more transition regions of a microwave energy steppedimpedance directional coupling device, which has plural cascaded quarter-wavelength sections of parallel-coupled transmission lines, is or are provided with sloped edges. The sloped edges of a transition region are disposed in an oblique relationship with respect to each other.

United States Patent In ventor Elmer E. Beech Apalachin, N .Y.

Appl. No. 854,360

Filed Aug. 27, 1969 Patented Nov. 2, 1971 Assignee International Business Machines Corporation Armnnk, N.Y.

STEPPED-IMPEDANCE DIRECTIONAL COUPLER 8 Claims, 6 Drawing Figs.

U.S. Cl 333/10, 333/34, 333/84 M Int. Cl 1101 3/08, HOlp 5/14 Field 0! Search 333/10, 84,

References cmii OTHER REFERENCES Tandem Couplers and Phase Shifters for Multi-Octave Bandwidth Shelton et al. in Microwaves April 1965 pages l4- 19 Primary Examiner- Eli Lieberman Assistant Examiner Marvin Nussbaum Attorneys- Hanifin and Jancin and Norman R. Bardales ABSTRACT: One or more transition regions of a microwave energy stepped-impedance directional coupling device, which has plural cascaded quarter-wavelength sections of parallelcoupled transmission lines, is or are provided with sloped edges. The sloped edges ofa transition region are disposed in an oblique relationship with respect to each other.

PATENTED NUV2 IHYI SHEET 1 OF 2 nndr STEPPED-IMPEDANCE DIRECTIONAL COUPLER BACKGROUND OF THE INVENTION This invention relates to directional couplers and more particularly to microwave energy directional coupling devices of the stepped-impedance type.

Microwave energy directional couplers of the stepped-impedance type are well known in the art and described, for example, in the literature such as: "Theory and Tables of Optimum Symmetrical TEM-Mode Coupled-Transmission-Line Directional Couplers," E. G. Cristal and L. Young, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-l3, No. 5, pages 544-558, Sept. l965;Characteristic Impedances of Broadside-Coupled Strip Transmission Lines," S. B. Cohn, IRE Transactions on Microwave Theory and Techniques, Vol. MTT-B, pages 633-637, Nov. 1960; "Impedances of Offset Parallel-Coupled Strip Transmission Lines, J. P. Shelton, Jr., IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-l4, No. 1, pages 7-15, Jan., 1966; to name just a few.

Heretofore, the prior art recognized that the shape ofthe transition region associated with a pair of adjacent quarterwavelength sections of the aforedescribed type of directional couplers effected the coupling between the adjacent sections; c.f., for example, the publication entitled The Design and Construction of Broadband, I-Iigh-Directivity, 90 Couplers Using Nonunifonn Line Techniques, C. P. Tresselt, IEEE Transactions on Microwave Theory and Techniques,

M'IT-l4, No. 12, pages 647-656, Dec., I966. Thus, as taught in the prior art, the edges of the transition region were generally provided in a parallel and/or colinear relationship with respect to each other. For example, generally the edges of the transition region between two adjacent quarterwavelength sections were provided with 45 miters, c.f. the article entitled "Tandem Couplers and PhaseShifters for Multi- Octave Bandwidth," J. P. Shelton, .I. Wolfe, and R. C. Van- Wagoner, Microwaves, pages 14-19, Apr., 1965; or perpendicularly aligned miters, c.f. the article entitled "Tables for Asymmetric Multi-Element Coupled-Transmission-Line Directional Couplers," R. Levy, IRE Transactions on Microwave Theory and Techniques, Vol. M'IT-l2, pages 27$-279,May, 1964.

While the use of such transition regions having the aforedescribed parallel and/or colinear edges'has resulted in improved coupling between the sections, they generally tended to have only a limited improvement in the ripple factor characteristic of the particular directional coupler.

SUMMARY OF THE INVENTION It is an object of this invention to provide a microwave energy directional coupler of the stepped-impedance type having an improved, is. attenuated, ripple factor characteristic.

It is another object of this invention to provide a stripline microwave energy directional couplerof the aforementioned kind.

According to one aspect of the invention, a microwave energy stepped-impedance directional coupling device, which has plural cascaded quarter-wavelength sections of parallelcoupled transmission lines, is provided with a transitionregion for at least one pair of adjacent sections that has sloped edges which are disposed in an oblique relationship with respect to each other.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view partially broken away of a preferred embodiment of the directional coupler of the present invention;

FIG. 2 is an enlarged cross-sectional view of the coupler of FIG, 1 taken along the line 2-2 shown in FIG. I, the vertical dimensions of FIG. 2 being exaggerated with respect to the horizontal dimensions thereof for sake of clarity;

FIG. 3a is an enlarged schematic diagram of thetransmission line conductors of the embodiment of FIG. 1;

FIGS. 3b -3c are enlarged partial schematic diagrams of the transmission line conductors of certain directional couplers of the prior art; and

FIG; 4 are idealized waveforms of the coupling response characteristic of a typical directional coupler'of the present invention.

In the Figures, like elements are designated with similar reference numbers.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1-2, there is shown a preferred embodiment of the microwave energy stepped impedance directional coupling device 10 of the present invention. Device or coupler 10 has plural cascaded quarter-wavelength sections of parallel-coupled transmission lines ll, 12.

In the preferred embodiment, the directional coupler 10 b. configuredas a stripline type. Accordingly, each of the transmission line conductors l1 and 12 have a flat or planar configuration. The conductors 11 and 12 are disposed in parallel planes with a predetermined spacing S. A pair of spaced ground plane members l3, 14, which also have respective planar configurations, sandwich the conductors ll, 11 in the spacing b. Conductors ll, 12 are preferably disposed with their planar configurations parallel to t'he planes of the ground plane members 13, 14. However, as is apparent to those skilled in the art, the conductors l1, l2'may be disposed with their respective planar configurations normal to the respective planes of the members 13, 14.

In the preferred embodiment, a solid dielectric means 15-17 is provided between the spaced ground plane members 13, I4 and spaced conductors ll, 12. More particularly, as shown in FIG. 2, the transmission lines l1, 12 are supported by an inner dielectric planar member 16. The remote outer surfaces of the conductors l1 and 12 are disposed adjacent to the outer dielectric planar members 15 and 17, respectively.

In FIG. 2, as aforementioned, the vertical dimensions are illustrated exaggerated relative to the horizontal dimensions for purposes of clarity. As a result, the respective vertical spacings shown between the remote outer surfaces of the conductors ll, 12 and the members 15 and 17, respectively, are greatly exaggerated and it should be understood that in practice those spacings would be negligible and/or practically nonexistent. In the preferred embodiment, the conductors ll, 12 are preferably formed on the center dielectric member [6 by a printed circuit process or the like. In this manner, the conductors ll, 12 are maintained in a permanent registration or alignment with respect to each other. Alternatively, the conductors ll, 12 may be formed on the respective inner surfaces of the members 15 and 17, respectively, and auxiliary registration means such as alignment pins may be provided on the members 15 and 17 to keep the conductors in the desired aligned relationship with respect to each other. In the aforedescribed cases, the solid dielectric means 15-17 also provide the means for supporting the conductors I1 and 12. It should be understood that the invention may alternatively be practiced with a gaseous type dielectric such as air, for example, in which case independent supporting means would be provided. In the preferred embodiment, the solid dielectric member's 15-17 are made of polyol'efm.

The ends of conductor 12 are connected to respective coaxial connectors 18, 19 and the conductor 12 functions as the through conductor. One end of conductor 11 is connected to a coaxial connector 20. The other end of conductor 11 is connected by solder, not shown, or the like, to a terminating impedance such as the resistor 21 via the latters center conductive tap or terminal 21a in a manner and for purposes well known to those skilled in the art. The conductor 11 functions as the coupling conductor. The connectors 1B 19, 20 have the protruding ends of their respective center conductors 18a, 19a, 20a shaped and connected by solder, or the like, to the appropriate ends of conductors 11, 12.

Annular openings 22 are provided in the members 15-17 for housing the cylindrical shaped resistor 21. A pair of annular spring-plate like conductive members 23, 24 are juxtaposed between the end surfaces of the resistor 21 and the cylindrical disk-shaped conductive members 25, 26. Members 25, 26, are seated in annular recesses provided in the members 13 and 14, respectively, for this purpose. Each of the two end surfaces of resistor 21 is a terminal. As a result, two symmetrical conductive paths connecting the conductor 11 to the ground planes 13, 14 are provided via the resistors center tap 21a and end terminals. Resistors of the type employed for resistor 21 are well known in the art and a typical commercially available resistor suitable for this purpose is referred to by the manufacturer as a "pill resistor or termination. In the preferred embodiment, a conventional microwave 50 ohm termination is used.

The sides of assembly 11-17 are inserted and mounted in the U-shaped channel formed in the conductive side members 27, 28. Assembly 11-17 is secured thereto by suitable means such as the machine screws 29 which pass through the members 13-17 which are appropriately bored, the channel members being appropriately threaded to receive the screws 29. In a similar manner, the ends of the assembly 11-17 are inserted and mounted in the respective U-shaped channel portions 30 formed in the end conductive members 31, 32 and secured thereto by machine screws 29'. Again, the members 13-17 are provided with appropriate bore holes and the U-shaped portions 30 of the members 31, 32 appropriately threaded for securing the screws 29'. The members 27, 28 and 31, 32 are countoured for compatible mating and thus provide a shielding enclosure which prevents leakage of the microwave energy from the sides or ends of the coupler 10. Screws 29, 29' are also judiciously spaced to prevent or mitigate any unwanted or spurious reflections of microwave energy in the coupler as a result of their presence. Members 27 and 28 are also threaded to receive the machine screws, e.g. screws 18. These screws secure the respective connectors 18-20, and more particularly, the ground conductor housings thereof, to the members 27 and 28 and hence connect the ground plane members 13 and 14 thereto.

In order to facilitate the insertion of the assembly 11-17 in the members 27, 28, 31, 32, the assembly 11-17 is held together by a pair of bolts 33 and nuts 34. A pair of hollow bolts 35, which are secured to the assembly 11-17 by the nuts 36, may be provided for mounting the device 10 to an equipment frame, not shown. A pair of threaded pins, not shown, which are mounted on the frame, are adapted to pass through the hollow bolts 35. A pair of nuts, not shown, are fitted to the pins, thus securing the device 10 to the frame. For the purpose of passing bolts 33 and 35 through the assembly 11-17, members 13-17 are provided with compatibly bored holes.

Referring now to FIG. 3a, the through conductor 12 is illustrated in solid line form and superimposed over the coupled conductor 11 shown in dash line form for sake of clarity. In the preferred embodiment, the directional coupler 10 is comprised of nine stepped-impedance quarter-wavelength sections designated Z1, Z2 Z9, respectively. It should be noted that each quarter-wavelength section includes parallelcoupled portions of conductors 11 and 12. In the preferred embodiment, the conductors l1 and 12 are symmetrically configured. Moreover, the center portions of the conductors 11, 12 corresponding to the center section Z5 are broadside coupled, whereas the portions of the conductors 11, 12 of any particular one of the other sections are offset coupled. For a given center frequency fc and corresponding wavelength he the dimension Ac/4 is obtained. As is well known to those skilled in the art for a given bandwidth ratio, center frequency, desired ripple factor and decibel coupling value, the widths S1. S2, S9 of the conductors 11, 12 as well as the offset dimension WC1-WC4, WC6-WC9, and the aforementioned spacing dimensions S and b, WC6-WC9, FIG. 2 are readily calculated. For example, it can readily be shown that the aforementioned dimensions may be obtained from the first three aforementioned publications by Crystal and Young, Cohn, and Shelton, Jr. The symmetrical configuration shown in FIG. 3a has the following relationships: S1=S9, S2=S8, S3=S7, S4=S6; S1 82 83 84 85; and WC1=WC9, WC2-WC8, WC3=WC7, WC4=WC6, and WC5=0.

Referring briefly to FIGS. 3b and 3c, heretofore in the prior art, the transition region between a pair of adjacent steppedirnpedance quarter-wavelength sections had edges 37 which were in a colinear relationship with respect to each other as shown in FIG. 3b or in a parallel relationship with respect to each other as shown in FIG. 3c. More particularly; with respect to FIG. 30, the angles Band 02 were made equal and preferably were 45, as aforementioned.

Referring again to FIG. 3a, according to the present invention, in the transition region between at least one pair of adjacent quarter-wavelength sections, each of the edges 37 of the particular transition region are disposed in an oblique relationship with respect to the other edges 37 of the particular region. In the preferred embodiment, each of the transition regions of each pair of adjacent quarter-wavelength sections has its sloped edges configured in the aforedescribed oblique relationship with respect to each other.

According to one manner of practicing the invention, the horizontal dimension L of the transition region is detennined from the equation:

where K is a constant equal to 0.14810025; The horizontal dimension L is symmetrically disposed about and is at right angles to the particular terminating line 38 which is common to the particular pair of adjacent quarter-wavelength sections. Each sloped edge 37' is then configured to pass through the center point of the particular vertical dimension, e.g. H1 or H2, which lies between the two particular horizontal parallel edges of the adjacent quarter-wavelength sections which are to be connected by the particular sloped edge. As is obvious to those skilled in the art, due to the differences in the width dimensions; e.g., S2 and S3, and the differences in the offset dimensions; e.g., WC2 and WC3, provided for the different pairs of adjacent quarter-wavelength sections; e.g. Z2 and Z3, it can be readily demonstrated that the corresponding vertical dimensions; e.g. H1 and H2, are not equal. Consequently, the sloping edges 37' pass through the respective midpoints of the vertical dimensions; e.g. H1 and H2, at different angles; i.e. 01 is not equal to 02.

As shown in FIG. 3a, those portions of the conductors 11 and 12 associated with the end quarter-wavelength sections Z1 and Z9 are connected to vertical portions 39. The latter have conventional and 45 miter corners connecting the particular vertical portions 39 with the particular horizontal portions of the end sections Z1 and Z9.

Referring now to FIG. 4 there is shown typical coupling response characteristic curves Ia, lb of the coupled conductor or arm 11 and the through conductor or arm 12, respectively, of the device 10 and the improved ripple characteristics Aa and Ab, respectively. In a particular nine-section stepped-impedance quarter-wavelength directional coupler built in accordance with the principles of this invention, the coupled arm had a measured response of -8.7:t0.7 decibels and a through arm response of --l .1103 decibels gI'Iz., a broadband width of two to 12 gI'Iz., i.e. %Aa=0.7 and l6Ab=0.3 decibels, respectively. For purposes of comparison, there is illustrated in FIG. 4, the corresponding response characteristic curves Ila, IIb shown in dashline form for a comparable nine-section stepped-impedance quarter-wavelength directional coupler which did not employ sloping edges in the transition regions in the manner taught by the present invention.

In operation, microwave energy is preferably fed into one port; i.e. connector 18, and is transmitted out the other ports; i.e. connectors 19,20.

The present invention, it should be understood is applicable to thin or thick strip conductors ll, 12, and/or to solid or gaseous type dielectrics as aforementioned. Thus, while the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

lclaim:

1. In a microwave energy stepped-impedance directional coupling device having plural cascaded quarter-wavelength sections of parallel-coupled transmission lines, the transition region of at least one pair of adjacent said sections being comprised with sloped edges, said edges being disposed in an oblique relationship with respect to each other.

2. A microwave energy stepped-impedance directional coupling device according to claim 1 wherein said device is of the stripline type.

3. A microwave energy stepped-impedance directional coupling device according to claim 1 wherein the transition region has a horizontal dimension parallel to the longitudinal direction of the parallel-coupled transmission lines, said horizontal dimension being substantially equal to L=K x )tc/4, where K is a constant equal to 0.l48fl.02$; and Ac is the wavelength of the center frequency of said device.

4. A microwave energy directional coupling device comprising:

a predetermined number of plural cascaded quarterwavelength sections of parallel-coupled first and second transmission line conductors of the stepped-impedance yp first and second ground plane members having first and second opposed planar surfaces, respectively, disposed in substantially parallel relationship with respect to each other;

means for supporting said first and second conductors in first and second parallel planes, respectively, between said first and second surfaces;

predetermined dielectric means disposed between said first and second ground plane members, said first and second conductors being disposed within said dielectric means; and

means coupled to said ground plane members and said first and second transmission line conductors for conducting microwave energy to and from said device; the transition region of at least one pair of adjacent said sections being comprised with sloped edges, said edges being disposed in an oblique relationship with respect to each other.

5. A microwave energy directional coupling device according to claim 4 wherein said device is of the stripline type.

6. A microwave energy directional coupling device according to claim 4 wherein the transition region has a horizontal dimension parallel to the longitudinal direction of the parallelcoupled transmission lines, said horizontal dimension being equal to where K is a constant equal to 0.l48i0.025; and Ac is the wavelength of the center frequency of said device.

7. A microwave energy directional coupling device according to claim 4 wherein said predetermined dielectric means are of the solid type.

8. A microwave energy directional coupling device according to claim 7 wherein said means for supporting and said predetermined dielectric means are the same.

i i I i 

1. In a microwave energy stepped-impedance directional coupling device having plural cascaded quarter-wavelength sections of parallel-coupled transmission lines, the transition region of at least one pair of adjacent said sections being comprised with sloped edges, said edges being disposed in an oblique relationship with respect to each other.
 2. A microwave energy stepped-impedance directional coupling device according to claim 1 wherein said device is of the stripline type.
 3. A microwave energy stepped-impedance directional coupling device according to claim 1 wherein the transition region has a horizontal dimension parallel to the longitudinal direction of the parallel-coupled transmission lines, said horizontal dimension being substantially equal to L K x lambda c/4, where K is a constant equal to 0.148 + or - 0.025; and lambda c is the wavelength of the center frequency of said device.
 4. A microwave energy directional coupling device comprising: a predetermined number of plural cascaded quarter-wavelength sections of parallel-coupled first and second transmission line conductors of the stepped-impedance type; first and second ground plane members having first and second opposed planar surfaces, respectively, disposed in substantially parallel relationship with respect to each other; means for supporting said first and second conductors in first and second parallel planes, respectively, between Said first and second surfaces; predetermined dielectric means disposed between said first and second ground plane members, said first and second conductors being disposed within said dielectric means; and means coupled to said ground plane members and said first and second transmission line conductors for conducting microwave energy to and from said device; the transition region of at least one pair of adjacent said sections being comprised with sloped edges, said edges being disposed in an oblique relationship with respect to each other.
 5. A microwave energy directional coupling device according to claim 4 wherein said device is of the stripline type.
 6. A microwave energy directional coupling device according to claim 4 wherein the transition region has a horizontal dimension parallel to the longitudinal direction of the parallel-coupled transmission lines, said horizontal dimension being equal to L K X lambda c/4, where K is a constant equal to 0.148 + or - 0.025; and lambda c is the wavelength of the center frequency of said device.
 7. A microwave energy directional coupling device according to claim 4 wherein said predetermined dielectric means are of the solid type.
 8. A microwave energy directional coupling device according to claim 7 wherein said means for supporting and said predetermined dielectric means are the same. 